Your Bloodwork Says Normal. Here's What It Missed.

One of the biggest myths about RED-S is that routine bloodwork will identify it. It usually doesn't. And that gap — between what standard testing finds and what's actually happening — is exactly why so many athletes spend months or years being told everything is fine while their performance keeps declining.

In the previous two articles (hereand here) in this series, we established what RED-S is and why it produces such a broad, systemic set of consequences. This article answers the question that follows naturally: how do you actually know if you have it?

The honest answer is that identifying RED-S requires a different kind of investigation than most athletes have ever had. Not because the condition is rare or exotic — it's neither — but because the markers that reveal it aren't part of standard medical workups, and the ones that are included are frequently interpreted through a disease-screening lens rather than a performance lens. Those are two very different bars, and most athletes fall into the gap between them.

Prefer to watch? The full video is below — or keep reading for the expanded breakdown.

It Starts With History

Before any lab is ordered, the most important diagnostic tool is a thorough history. Not a standard intake form — a detailed conversation about training load, dietary patterns, body composition goals, performance trajectory, and symptom timeline.

The pattern worth listening for is specific: an athlete who is training consistently, eating what they consider a healthy and disciplined diet, and experiencing a cluster of symptoms — fatigue that doesn't resolve, declining performance, slow recovery, mood changes, recurring minor injuries — that don't have an obvious explanation and haven't responded to the usual adjustments.

The cultural context matters too. Athletes who have adopted low-carbohydrate approaches for body composition or metabolic health. Athletes using intermittent fasting or extended eating windows. Athletes who are consciously trying to stay lean as they age and have gradually reduced caloric intake without adjusting for training demand. None of these are inherently problematic — but in the context of a high training load, any of them can create the chronic energy gap that drives RED-S without the athlete having any intention of underfueling.

The symptom timeline is also diagnostically meaningful. RED-S doesn't develop overnight. It develops over months, sometimes years, of chronic energy insufficiency. An athlete whose symptoms have been slowly worsening over an extended period — in the context of sustained training and disciplined eating — fits the RED-S pattern in a way that acute illness or simple overtraining typically doesn't.

Estimating Energy Availability

Before getting to labs, there's a clinical tool worth using directly: estimating energy availability.

Energy availability is calculated as energy intake minus exercise energy expenditure, divided by lean body mass. The threshold the research consistently identifies as problematic is below 30 kilocalories per kilogram of lean body mass per day. Below 45 is considered suboptimal. And the challenging reality is that many athletes with RED-S are operating in the 20 to 30 range — sometimes lower — without realizing it.

Getting an accurate estimate requires dietary recall data and training load information. It's not perfectly precise, but even a rough calculation can be illuminating. An athlete training 10 to 12 hours per week at meaningful aerobic intensity who is consuming 1,800 to 2,000 calories daily is almost certainly operating at a significant deficit relative to lean body mass — regardless of how nutritious that diet is in terms of food quality.

This estimate, used alongside history and lab work rather than as a standalone diagnostic, is often the number that makes the picture click for an athlete who has never connected their caloric intake to their actual training energy cost.

The Testing Panel That Actually Reveals RED-S

Ferritin — More Than an Iron Marker

Ferritin is the stored iron marker, and as covered in the lab testing article earlier in this series, it must be ordered separately from a standard iron panel. In the context of RED-S specifically, ferritin matters for two reasons.

First, iron deficiency is genuinely common in endurance athletes with RED-S — particularly female athletes — because dietary iron intake is frequently insufficient alongside the other dietary gaps that characterize this condition. Low ferritin directly impairs oxygen delivery and aerobic capacity, compounding the fatigue and performance decline that RED-S produces through hormonal mechanisms.

Second, ferritin is an acute phase reactant — it rises with systemic inflammation. In athletes with the chronic low-grade inflammation characteristic of RED-S, ferritin can be falsely normal or even falsely elevated, masking actual iron deficiency. This is why ferritin needs to be interpreted alongside other inflammatory markers and the full clinical picture, not in isolation.

For endurance athletes, the target is ferritin above 50 ng/mL, and ideally closer to 75 to 100. The standard lab reference range — anything above 12 or 15 — is not the right bar for this population.

Testosterone, Free Testosterone, and SHBG

Total testosterone alone is insufficient — always. The complete picture requires total testosterone, free testosterone, and SHBG together, because SHBG determines what fraction of total testosterone is actually bioavailable to tissues.

In athletes with RED-S, the most common pattern is total testosterone that looks marginal or low-normal — not dramatically deficient, just not where it should be for someone training at this load — alongside SHBG that is elevated, further suppressing the free fraction. The total number looks passable on a standard report. The free testosterone actually available to drive adaptation is significantly lower.

This applies equally to men and women. In women, testosterone is naturally lower in absolute terms, so even small drops in the free fraction are functionally significant. In women with RED-S who are also perimenopausal, the training-driven suppression compounds the natural hormonal transition — frequently in ways that get attributed entirely to age rather than to the addressable, energy-deficiency-driven component layered on top of it.

Estradiol

In women, estradiol gives direct insight into ovarian function and the downstream hormonal support for muscle repair, bone maintenance, and neurotransmitter regulation. Low estradiol in a premenopausal woman experiencing menstrual disruption is a straightforward finding. But as noted in the previous article, some women with RED-S maintain apparently regular cycles even with significantly suppressed estradiol — which is why testing matters rather than using menstrual status as a proxy.

In perimenopausal women, interpreting estradiol requires understanding where she is in the natural trajectory of the transition — looking for levels that are lower than expected for that stage, rather than simply a number below a general reference range.

In men, estradiol matters because it reflects testosterone-to-estrogen conversion through aromatase — and when testosterone is suppressed in RED-S, estradiol declines with it, removing its support for bone health and cardiovascular function in ways that rarely get identified because estrogen testing in men is widely assumed unnecessary.

Cortisol Rhythm

A single cortisol blood draw is not clinically useful here — for all the reasons covered throughout this series. What's useful is a multi-point salivary cortisol test mapping the full diurnal pattern: four samples across the day, from early morning through evening.

In athletes with RED-S, one of two patterns typically emerges. The first is chronically elevated cortisol across the full curve — the system has been in a sustained stress response state, consistent with earlier or less severe RED-S. The second is a flattened or blunted pattern — cortisol that can no longer generate appropriate morning peaks — reflecting more prolonged adrenal compromise. That second pattern is more concerning and typically associated with a longer recovery timeline.

DHEA-S

As covered in the mechanism article, DHEA declines in parallel with cortisol-driven suppression of adrenal anabolic function. DHEA-S — the sulfated storage form — is the most reliably measured version in blood, and it's absent from most standard hormone panels.

Low DHEA-S in the context of the other findings on this panel confirms that adrenal compromise is significant — that the body has been prioritizing stress response over anabolic output for a sustained period. Because DHEA naturally declines with age more rapidly than most other hormones, this finding is particularly common and particularly significant in masters athletes, where the baseline reserve is already reduced before RED-S further depletes it.

The cortisol-to-DHEA ratio is clinically meaningful here: high cortisol relative to low DHEA reflects a system heavily weighted toward catabolism and away from the anabolic hormonal environment that recovery and adaptation depend on.

Complete Thyroid Panel: TSH, Free T4, and Free T3

TSH alone is not a thyroid panel — not for athletes, and not in the context of RED-S.

As covered in detail in the hormone suppression article, T4-to-T3 conversion is suppressed in RED-S as a deliberate energy conservation strategy. That suppression is invisible to TSH, which reflects pituitary signaling about T4 production only.

The pattern to look for in RED-S is: normal or low-normal TSH, normal Free T4, and reduced Free T3. Normal at the production level. Impaired at the conversion step. That combination is the thyroid fingerprint of chronic energy deficiency in athletes — and it's missed by standard testing because Free T3 is almost never included in a routine panel.

Vitamin D

Vitamin D supports bone metabolism alongside estrogen and testosterone, modulates immune function, and deficiency is independently associated with impaired muscle function, increased injury risk, and reduced performance. In athletes with RED-S already dealing with bone remodeling disruption and immune compromise, vitamin D insufficiency compounds both consequences.

The performance target is 50 to 80 ng/mL — not the standard sufficiency threshold of 20 ng/mL, which is calibrated for disease prevention in a general population rather than athletic performance.

Complete Blood Count

A CBC adds useful texture to the overall picture. Red blood cell indices — mean corpuscular volume in particular — can reflect early changes in B12 or folate status that accompany chronic dietary restriction. White blood cell differential can show patterns consistent with chronic stress or immune suppression. Hemoglobin, while less sensitive than ferritin for early iron deficiency, provides context for the ferritin finding when it's borderline.

CBC isn't a primary diagnostic for RED-S, but it contributes to the pattern — particularly when other findings are equivocal.

Bone Density (DEXA)

DEXA scanning for bone mineral density isn't part of the initial workup in every athlete — but it becomes important when there's a history of stress fractures, when RED-S is suspected to be longstanding, or when the athlete is post-menopausal or significantly estrogen-deficient.

Bone density loss in RED-S is silent until a fracture occurs, and by the time a fracture happens, meaningful loss has often already accumulated over months or years. For masters athletes — where age-related bone loss is already occurring alongside RED-S-driven hormonal suppression of bone remodeling — the threshold for ordering a DEXA scan as part of the evaluation should be lower than it would be for a younger athlete.

Performance Clues That Complete the Picture

Lab work and history don't operate in isolation. There are performance markers that support the RED-S picture when present:

Heart rate variability trending downward over a sustained period. Resting heart rate gradually elevated above personal baseline. Power or pace at a given perceived effort declining without explanation. Heart rate higher than usual for the same training output — reflecting the cardiovascular cost of operating in an energy-deficient state. Recovery between sessions measurably slower than historical baseline.

None of these are diagnostic in isolation. But when they occur alongside the symptom pattern and lab findings above, they add clinical confidence that the picture is RED-S rather than simple overtraining or age-related decline.

Reading the Pattern, Not the Numbers

The diagnosis of RED-S isn't a single lab value. It's a constellation of history, energy availability estimate, hormonal markers, nutrient status, and performance data that collectively points to chronic energy insufficiency as the underlying driver.

The patterns that are most diagnostically compelling:

• Suppressed total and free testosterone alongside elevated SHBG
• Low DHEA-S alongside dysregulated cortisol — particularly the cortisol-to-DHEA ratio
• Reduced Free T3 with normal TSH and Free T4
• Low or functionally insufficient ferritin
• Vitamin D below the performance threshold
• A history of sustained high training load alongside disciplined eating that isn't calibrated to that load

When those findings occur together — and they frequently do, because they all share the same upstream cause — the picture is RED-S until proven otherwise.

And when an athlete has been told their standard labs are normal while experiencing all of this — the most common presentation — it's not that nothing is wrong. It's that the right things weren't tested, or weren't interpreted through the right lens.

What Comes Next

Identifying RED-S is the necessary first step. What to actually do about it — and why the recovery approach matters as much as the diagnosis — is where most athletes make a critical mistake. The next article in this series covers what actually gets you back to full performance, and why attempting to train through RED-S consistently makes it worse rather than better.

Dr. Jason Barker is a naturopathic doctor with over 20 years of clinical experience working with endurance athletes. He is a two-time Ironman finisher and the founder of Natural Athlete Clinic. For individualized evaluation including the complete testing panel covered in this article, visit naturalathleteclinic.com.